Large power generators have water cooled stator windings. These windings require regular maintenance and testing for electrical and hydraulic leaks. These tests require the stator winding to be completely dry. However, it is extraordinarily difficult to dry stator windings.
Electrical tests, such as insulation resistance and electrical overpotential, can only be reliably undertaken when the stator is entirely free of moisture. The electrical test results will be inaccurate if the winding is damp. In addition, a water cooled stator receives hydraulic testing approximately every two and one/half years. Hydraulic tests detect water leaks within the stator winding by evaluating the ability of the winding to hold vacuum and pressure.
Prior methods of drying water-cooled stators and conducting hydraulic tests were difficult and time consuming. For example, the methods employed in a generator factory begin by draining the winding by gravity. However, about 10% to 30% of the water in the stator will not drain by gravity due to a manometer effect.
To remove the remaining water, an air hose is connected to the water inlet on the generator winding. The other end of the air hose is connected to an air manifold having an air actuated valve to control air flow. The manifold is itself connected to a receiver through which compressed air from the factory is supplied.
To dry the stator windings in the factory, shop compressed air first fills the receiver and manifold. Once the receiver is pressurized, the air actuated valve on the manifold is opened and compressed air flows into the generator, through the stator windings and out the open drain valve. The flow of air forces out some of the remaining moisture in the stator. After the compressed air exhausts from the receiver, the manifold valve is closed, and the receiver is again filled with compressed air to repeat the drying cycle. This cycle is repeated until all moisture is purged from the generator.
The factory method of drying a stator is disadvantageous because shop compressed air often has contaminants such as oil, water, and rust. These contaminants get caught in and can harm the stator windings. Moreover, moisture introduced into the stator by the shop air undermines the drying process. To purge all of the moisture from the stator, bottled nitrogen has been used to blow nitrogen through the stator windings for about an hour following the blow down with air. Bottled nitrogen is generally cleaner and dryer than the factory compressed air, and is useful for final drying. In addition, a vacuum has been created in the stator windings to boil off and draw out any remaining moisture.
In these prior methods, moisture in the air exhausting from the stator is visually monitored to determine when the stator is finally dry. However, no reliable method of evaluating the moisture in the exhaust air was available in the past. Because of the inherent inaccuracy in this method of determining stator dryness, procedures have been followed in the past in which air is blown through the stator for 12 to 16 hours before any hydraulic or electrical tests are attempted. These procedures are uncertain and wasteful. If the stator is dry in the first 8 or 10 hours, then the remaining time of the procedure is wasted and expensive down time. On the other hand, if the winding is not dry, then time can be wasted in attempting unsuccessful electrical and/or hydraulic tests.
Field testing and drying a generator is even more difficult than factory testing. Prior methods of drying water cooled stator windings in the field are similar to factory drying methods in that water in the stator is first allowed to drain out under the force of gravity. The compressed air available at the site, which often has more moisture and contaminants than does factory shop air, is connected to the inlet port piping on the generator.
In prior field methods, to pressurize the stator a valve on the outlet drain piping is manually closed. The shop air fills the stator winding. When a selected air pressure is reached in the winding, the outlet valve to the generator drain is manually opened to expel moisture out of and relieve the pressure in the winding to atmospheric pressure. Then, the outlet valve is again closed and the winding allowed to refill with pressurized shop air. Once the selected high pressure is reached, the outlet valve is again opened to expel moisture and exhaust pressure. This process is repeated until the exhaust gas appears to be free of moisture. However, the stator winding has a minimal internal passage volume and, thus, cannot hold much pressurized air. A large volume of pressurized air is needed to blow out the moisture in the stator. Accordingly, the air flow is reversed within the stator from time to time during this process by connecting the shop air to the outlet drain so as to purge moisture from the inlet header side of the winding.
This field method of purging moisture from a stator winding has the same disadvantages as does the method used in the factory. In addition, the field method lacks a compressed air receiver to store a large volume of compressed air that is repeatedly purged through the winding. In the field, the compressed air is stored only in the smaller volume of the stator windings. Thus, the amount of air passing through the winding during each cycle is less in the field than it is in the factory where a receiver is used. Moreover, air at the inlet end of the generator is not pushed completely through and out the exhaust end of the generator. Accordingly, a time-consuming procedure to reverse the air flow is needed in field operations to purge all moisture out of both ends (and throughout) of the stator windings.